An implantable cardioverter-defibrillator (“ICD”) is a medical device combining a cardioverter and a defibrillator into a single implantable unit. One type of ICD is implanted intravascularly or intracardially, having leads positioned within the heart as well as being attached to the outer surface of the heart. Such ICDs require the plurality of high energy capacitors to provide about 35 to 40 joules of energy per electrical impulse, as well as having a high voltage per electrical impulse in the range of 700 to 800 volts to properly defibrillate, for example, a patient suffering from ventricular fibrillation. Another kind of ICD is implanted subcutaneously and includes leads that are placed under the skin and without direct contact with the heart. Such ICDs are also known in the art as “subcutaneous ICDs.” The electrical impulse in a subcutaneous ICD needs to pass through muscles, the lungs and bones to defibrillate the heart as the leads are placed subcutaneously and are not in contact with the heart. As such, subcutaneous ICDs must employ even higher energy and voltage levels than intracardiac ICDs.
Wet electrolytic capacitors are used in most state of the art ICDs due to their ability to store high energy levels as well as their ability to maintain high voltages, and thus have a high volumetric efficiency. The most common capacitor shape used in ICDs is in the form of a half circle or “D shape.” Such D-shaped capacitors are not, however, suitable for use in many types of implantable subcutaneous devices due to their low ratio between capacitance and size. While cylindrically-shaped capacitors seem to be an ideal candidate, it has proven difficult to use such capacitors as the cylindrically-shaped anode increases the overall distance between any point in the anode to any point in the cathode. This increase in distance causes an increase in the inner resistance of such a capacitor, thereby increasing the equivalent series resistance (herein abbreviated ESR) for such capacitors. The increase in ESR would thus not enable sufficient energy to be stored and delivered to the heart of a patient subcutaneously given the size and volume constraints.
As such, a need still exists for an improved cylindrically-shaped wet electrolytic capacitor for use in subcutaneous ICDs.